1. Field of the Invention
The present invention relates to a high voltage generator for generating a high voltage higher than a power-supply voltage by a predetermined potential in order to supplement a loss of a threshold potential of a transistor in a semiconductor memory device. More particularly, it relates to a high voltage generator for ensuring a reduction of a current consumption in a standby mode as well as a high-speed operation in an active mode.
2. Description of the Prior Art
In general, a high voltage (hereinafter referred to as `Vpp`) generator provides a constant high voltage to a circuit of chip requiring a voltage higher than a power-supply voltage Vcc, and is widely employed to a memory circuit field such as a DRAM, especially to a word line driver and a bit line division circuit and a data output buffer, etc.
FIG. 1 is a circuit diagram of a conventional high voltage generator.
As shown in FIG. 1, the conventional high voltage generator includes: potential sensing units 10 and 12 which are selectively enabled in a standby mode and an active mode, sense a high voltage (Vpp) potential level in each mode, and generate potential sensing signals det1 and det2; delay units 20 and 22 for delaying the potential sensing signals det1 and det2 from the potential sensing units 10 and 12 by a predetermined time; oscillation units 30 and 32 which receive the potential sensing signals det1 and det2 of each mode (i.e., standby mode and active mode) from the delay units 20 and 22, are driven by the potential sensing signals det1 and det2, and generate a pulse signal with a predetermined period in order to operate pumping units 40 and 42; and pumping units 40 and 42 for performing a potential pumping operation according to the pulse signal from the oscillation units 30 and 32, and adjusting a potential of a high voltage.
The potential sensing unit 10 for sensing a high voltage Vpp potential level in the standby mode includes: two PMOS transistors MP11 and MP12 having gate terminals commonly grounded, the PMOS transistor MP11 being connected between a power-supply voltage Vcc terminal and a node N11, and the other PMOS transistor MP12 being connected between a high voltage Vpp terminal and a node N12; two NMOS transistors MN11 and MN12 whose gate terminals are commonly connected to the node N11; and an inverter IV11 connected to the node N12.
At this time, since the potential sensing unit 10 is operated in a standby mode, the potential sensing unit 10 has a low current consumption and a slow operation speed.
The pumping unit 40 used for a potential pumping operation in the standby mode employs a low driving power, because consuming a high voltage Vpp in the standby mode is based on a leakage current of a very small quantity.
In the meantime, the potential sensing unit 12 for sensing a high voltage Vpp potential level in active mode includes: two PMOS transistors MP13 and MP14 having gate terminals commonly grounded, the PMOS transistor MP13 being connected between a power-supply voltage Vcc terminal and a node N13, and the other PMOS transistors MP14 being connected between a high voltage Vpp terminal and a node N14; two NMOS transistors MN13 and MN14 having gate terminals commonly connected, which are respectively connected to the two nodes N13 and N14; two NMOS transistors MN15 and MN16 which are respectively connected between the two NMOS transistors MN13 and MN14 and a ground terminal, and have gate terminals receiving an operation control signal act of informing an active mode's start; and an inverter IV12 connected to the node N14.
Since the potential sensing unit 12 is operated in active mode, the potential sensing unit 12 should be designed to have an operation speed of high speed occurring a current consumption of high value. The pumping unit 42 used in active mode has a plurality of circuits for performing a charging/precharging operation by using a high voltage Vpp, so that it should have a high driving power.
FIG. 2 is a timing diagram of a high voltage generator shown in FIG. 1.
Although there is a time lag between the pumping unit 42 enabled in active mode and the pumping unit 40 enabled in a standby mode, the pumping unit 42 enabled in active mode and the pumping unit 40 enabled in a standby mode are driven by the same method, so that a description about one mode will be discussed.
If a high voltage Vpp shown in FIG. 2(a) is lower than a reference potential V.sub.target, the potential sensing unit 10 or 12 changes a state of a potential sensing signal det1 or det2 to a logic low state as shown in FIG. 2(b) in order to operate a pumping unit 40 or 42, thus generates a logic low signal.
Then, the potential sensing signal det1 or det2 passes through a delay unit 20 or 22, and is delayed by a predetermined time t1. Accordingly, the oscillation unit delays a pump enable signal ppe for driving a pumping unit by the predetermined time t1 as shown in FIG. 2(c), a then generates the pump enable signal ppe delayed.
The pump enable signal ppe drives the pumping units 40 and 42, thereby achieving a potential pumping operation of a high voltage Vpp. Accordingly, if the potential level of a high voltage Vpp reaches to the target value V.sub.target, a potential sensing unit senses the high voltage potential level reached to the target value V.sub.target, and changes a state of the pump enable signal ppe to a disable state so as to stop a pumping operation.
However, the potential sensing signal det1 or det2 generated to stop the pumping operation is delayed by a delay unit by a predetermined time t1, and is then transmitted to the oscillation units 30 and 32. After elapsing a time period tl from a time point at which the potential sensing units 10 and 12 sense that a potential level of high voltage Vpp is beyond the target value V.sub.target, the oscillation units 30 and 32 changes a state of a pump enable signal ppe for controlling a pumping unit, and thereby generating a pump enable signal ppe having a changed state.
As a result, although the potential level of high voltage Vpp reaches to a predetermined level V.sub.target, unnecessary pumping operation is continuously performed for the time period t1 made by the delay unit, thereby raising a potential over the target value V.sub.target.
After the time period t1, the pumping unit stops operating by the transition pump enable signal ppe, then the potential level of high voltage Vpp is slowly lowered by a leakage current. At this time, if the potential level of high voltage Vpp is lower than the target value V.sub.target, the aforementioned operations are repeated again, thereby performing a charge pumping.
In this case, since the pumping operation is delayed by the time period t1, the potential level of high voltage becomes lowered than the reference potential value V.sub.target during the time period t1.
Accordingly, the conventional art which performs a charge pumping operation by using a predetermined delay time made by the delay unit in order to control a potential level can maintain a high voltage Vpp of a stable and constant potential level in case of a small quantity of current consumption. But, if an active mode requiring a high-speed operation starts as an abrupt current consumption by operations of a word line and peripheral circuits occurs, a response speed is greatly slowed down, thereby making it impossible to perform a stable operation.
In addition, if a pump driving power raises to correspond to much current consumption and a pumping operation is performed for a predetermined time, the conventional art may greatly change a potential level of high voltage in case of a little current consumption.
Namely, the conventional high voltage generator performs a stable operation without changing a high voltage potential level in case of a little leakage current. But, in case of active mode, it has a slowed response speed and raises a trembling degree of a potential level.